A "ship of the line" was a large, powerful ship designed for the line-of-battle tactic which mostly required opposing fleets to form single-file lines in the same tack and batter each other until one or the other was beaten. Lighter, less powerful ships weren't included in this.

"Man of war" is a more generic term over several centuries referring to sailing ships designed for combat instead of pure merchant business. This included the ship of the line when they evolved.